Why can I no longer easily navigate inside of this mesh?
You're likely in Orthographic view, rather than Perspective View. Press 5 on your numeric keypad to toggle between the two.

If for some reason you want to stay in orthographic view and still see the inside of the mesh.

Some options:


Hide faces that obscure the interior.
Use mask modifier on meshes to mask out some geometry.
Use view-clipping feature (&#9095; AltB)
Invert the normals of your mesh (&#9096; Ctrl&#8679; ShiftN) and enable backface culling in 3D view > Properties